Handle timeout in PostgreSQL::Test::Cluster::is_alive()

Handle timeout in PostgreSQL::Test::Cluster::is_alive()

This commit adds an extra --timeout=PG_TEST_TIMEOUT_DEFAULT to the call
of pg_isready done in is_alive(), so as it is possible to have more
leverage with the call on machines constrained on resources.

By default the timeout is 180s, and it can be changed depending on the
environment where the tests are run.

Per buildfarm member mamba, where the default timeout of 3s used by
pg_isready has proved that it may not be enough as the postmaster may
not have the time it needs to reply to a ping request.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/29b637df-f818-4b52-986a-f11ba28300e9@gmail.com